All of the hotel packages offered by Disneyland Paris can be found here:
https://www.disneylandparis.com/en-gb/offers/
As there are no ferries between Northern Ireland (or the Republic) and France, your main travel options would be as folows:
Route 1(a):
easyJet from Belfast International to Paris CDG.
TGV train to Marne La Vallée–Chessy (which is the station serving Disneyland Paris and is located within the grounds of the site)
The TGV trains operate at around 200mph, so it's a very short journey (10 to 15 minutes) but you might have quite a long wait for a train at the airport.
Checking in at at Belfast International at around 1200, for the one flight per day at 1355, might see you arrive at Disneyland Paris at around 1800 (local time), so that's roughly a 5-hour journey from check-in until arrival.
Route 1(b):
As above but take the dedicated bus service from Paris CDG to Disneyland Paris. You might have a shorter wait for the bus than you would for the TGV train but it's a 90 minute journey on the bus, so that might stretch the overall journey time from check-in until arrival to around 6 hours.
Route 1(c):
As above but using the RER trains between Paris CDG and Marne La Vallée–Chessy. However you'd need to change trains en route (at Chatelet Les Halles station), with a total journey time of around 100 minutes, so it would probably be better to use one of the options above.
[Note: There are no flights to Paris from Belfast City Airport]
Route 2(a):
Stena Line from Belfast to Liverpool, using the overnight service.
Taxi to Liverpool Lime Street station.
Train to Euston.
Taxi to St Pancras International (or a short walk to Euston Square station and then one stop to Kings Cross/StPancras)
Eurostar to Paris Gare Du Nord.
RER train to Marne La Vallée–Chessy (changing at Chatelet Les Halles, with a total journey time of around 70 minutes).
Your total journey time via that route, fro check-in for the ferry to arrival would be around 20 hours (which is a very long time for an 8-year-old!)
Route 2(b):
As above but using the direct train service from St Pancras International to Marne La Vallée–Chessy. However Eurostar only operate one train per day on that route; to tie up with the overnight ferry from Belfast you'd end up having to kill around 5 hours at St Pancras International and your total journey time would still be approaching 20 hours.
[Note: The other ferry services from Belfast don't take you to anywhere near a fast train service to London]
Route 3:
It would also be possible to fly from Belfast to one of London's airports and then get to St Pancras International at a time which would be convenient for the daily direct train service with Eurostar to Marne La Vallée–Chessy. However I can't really work on that route without knowing the exact dates involved in order to see which flights would be best. (easyJet fly from Belfast International to Gatwick, Luton and Stansted. Ryanair fly from Belfast International to Stansted. Aer Lingus fly from Belfast City to Heathrow. BA fly from Belfast City to London City and Heathrow).
If I was taking an 8-year-old to Paris though, I wouldn't stay at one of the Disneyland hotels anyway. I'd stay in central Paris and then take a day trip to Disneyland. On a different day I'd show the youngster some of the sights of Paris, such as the Eiffel Tower and spend lots of time at the Jardin d'Acclimatation, which is far, far cheaper than Disneyland and just as much fun (if not more so) for young children:
https://www.jardindacclimatation.fr/en